1. Field of the Invention
The invention relates to a voice recording apparatus.
2. Description of the Related Art
Conventionally there are voice recording apparatuses of a type in which a voice signal produced by a microphone is converted to a digital signal and then stored in a semiconductor memory, and, at the time of playback, the voice signal is read from the memory and then converted to an analog signal, which, in turn, drives a loudspeaker to produce sounds.
As disclosed in Japanese Unexamined Patent Publication No. 63-259899, this type of voice recording apparatus is equipped with display means or indicating means that allows users to receive information about how much of the recording capacity of the semiconductor memory has been used or how much the recordable time is left. In addition, Japanese Unexamined Utility Model Publication No. 1-78400 discloses a voice recording apparatus which changes the mode of display when the remainder of the semiconductor memory recordable time reaches a predetermined limit.
The voice recording apparatuses of the type described above reduces the amount of data to be generated as much as possible by means of high-efficient encoding of digital voice signals, thereby reducing the amount of data to be stored in the semiconductor memory. Particularly in recent years, various voice encoding techniques have been developed with the advance of digital signal processing techniques, allowing the recordable time to be prolonged greatly.
High-efficient encoding techniques include fixedlength encoding techniques which allocate the same amount of information in a predetermined unit of time regardless of the voice state and variable-length encoding techniques which vary the amount of information to be allocated in accordance with the voice state. In the fixed-length encoding, the amount of information generated for the same sound quality generally becomes greater than in the variable-length encoding. To reduce the amount of information to a minimum, therefore, the variable-length encoding technique is used frequently.
To save the amount of data to be recorded on the semiconductor memory, the so called VAD (Voice Activity Detection) feature is used frequently, which makes a voiced/voiceless decision to an input voice signal from a microphone, stops the recording operation when a voiceless (unvoiced) interval lasts for a predetermined period of time, and restarts the recording operation upon receipt of a voiced input.
Generally, as the encoding bit rate is set lower, the amount of data becomes smaller, so that the recordable time increases, but conversely, the sound quality deteriorates. On the other hand, the sound quality becomes better as the encoding bit rate is set higher, in which case the amount of data increases to reduce the recordable time. For this reason, a voice recording apparatus has been proposed which allows the user to make a selection among different encoding bit rates and encodes a voice signal at a bit rate which is selected.
However, the variable-length encoding technique and the VAD feature, which are effective in reducing the amount of use of the memory, have a drawback in that since the amount of information varies, the amount of use of the memory per unit time cannot become constant and the time that the voice signal can be recorded in the memory cannot be known correctly. In addition, the conventional voice recording apparatus that encodes voice signals at a bit rate selected from among multiple encoding bit rates displays the remainder of the recordable time in terms of a count in a timer circuit and hence can accommodate only one bit rate. It therefore has a drawback in that, when another bit rate is selected, the remainder of the memory recordable time cannot be displayed correctly.